The field of the invention is the preservation of hypoxic tissue.
Heart disease is the number one killer of adults in the industrialized world. The majority of acquired heart disease is due to coronary artery disease, in which blood flow to an area of the heart is reduced or eliminated, resulting in death of myocardium and replacement with nonfunctional scar tissue (1, 2). Fatal outcomes are common for individuals suffering acute occlusion of a coronary artery, typically within the first 24 hours.
Hypoxic cardiac tissue post-infarction can be broadly divided into three distinct zones. The direct area of ischemia that has total loss of blood supply sustains largely irreversible cell death and scar tissue-formation. The myocardium immediately surrounding the infarct zone is less severely affected but remains hypoxic. In some cases, cellular changes occur in this area that decrease energy utilization and promote cell survival. This “hibernating myocardium” may eventually recover if neo-angiogenesis or redirection of blood flow restores supply of oxygen and energy substrates (3, 4). Finally, the remaining myocardium typically remains well oxygenated and initially free of damage. The expansion of cell death is a key feature of myocardial infarction as partially ischemic regions of the heart ultimately succumb to hypoxia and are also replaced by scar tissue.
Efficient methods to limit initial loss of myocardium and subsequent expansion of the infarct in the acute period could be of significant value. In fact, overexpression of the survival kinase Akt (protein kinase B) in mesenchymal stem cells injected into mouse hearts postinfarction resulted in a decrease in infarct size (5), possibly as a result of secreted factors from the cells introduced into the heart. Subsequently, work from our laboratory demonstrated that the 43-amino acid protein thymosin β4 activates Akt via integrin linked kinase (ILK) and dramatically protects bordering myocardium from cell death in the first 24 hours after coronary occlusion (6). Given the efficacy of this small protein in our experimental model and the possibility of bypassing hurdles associated with stem cell administration, we investigated the potential for other proteins that activate Akt and have angiogenic properties similar to thymosin β4 to provide beneficial effects post-infarction.
The secreted chemokine stromal cell-derived factor-1α (SDF-1α) and its G-protein-coupled receptor CXCR4 have been implicated in cardiogenesis. Signaling downstream of CXCR4 can trigger a chemotactic response resulting in migration towards an increasing SDF-1α gradient (7-10). In addition, in some cell types, CXCR4 signaling can result in activation of Akt and stimulation of cell proliferation, survival, and angiogenesis (11-17). SDF-1α is upregulated post-infarction (18), and when administered by gene therapy after myocardial infarction reportedly increases homing of bone marrow-derived cells to the area of infarct (19, 20). Itescu (US Pat Pub No. 2005/0233992) discloses administering an inhibitor of SDF-1 to treat myocardial ischemia (MI), and reports that injection of SDF-1 48 hours after triggering MI improved cardiac function through a direct mechanism which involves induction of cardiomyocytes cycling and regeneration and an indirect mechanism operating through enhanced chemotaxis of mobilized bone marrow-derived endothelial progenitors and cardiac neovascularization. Damas et al. reported decreased plasma levels of SDF-1α in patients with coronary artery disease presenting with ischemic chest pain, and suggested that SDF-1α may have a plaque-stabilizing effect and that therapeutic intervention that enhances SDF-1α activity could potentially be beneficial in acute coronary syndromes (Damas, 2002). It has also been reported that SDF-1 administered to an animal model of ischemic hind limb enhanced recruitment and incorporation of transplanted endothelial precursor cells to the ischemic tissue (Yamaguchi, 2003).
We have found that SDF-1α administered to tissue subject to hypoxia prior to hypoxia-induced cell death in the tissue, alters the metabolism of ischemic cells so that they can better withstand hypoxia and evade hypoxia-induced cell death. In contrast to prior work, our methods do not rely on cycling, regeneration, immigration or neovascularization. Rather, we have found a distinct effect, that proximately administered SDF-1α alters the metabolism of ischemic cells so that they can better withstand hypoxia. Our cardioprotective effect is observed within 24 hrs of the onset of hypoxia; hence, in our methods the SDF-1α must be administered prior to hypoxia induced cell death, and prior to the signaling events that lead to cycling, regeneration, etc.